In the hydrogen peroxide process, aluminium oxide is used for regenerating and removing organic secondary products formed in the working solution. The porosity of active aluminium oxide allows it to be used particularly for the purpose of adsorbing organic compounds used in the anthraquinone process. The impurities are mostly aromatic hydrocarbons. Having lost its activity, aluminium oxide is removed from the process and replaced with fresh oxide. The spent aluminium oxide contains organic substances which restrict its utilization. At present, the waste aluminium oxide is usually disposed of as waste, or stored with a view of potential utilization.
There have been various attempts to purify impure aluminium oxide. The applicant's previous patent application PCT/FI95/00273 discloses a method based on heat treatment. According to the method, spent aluminium oxide from an anthraquinone working solution is heat treated at a temperature of 500-900.degree. C. The heat-treated aluminium oxide is dissolved in a concentrated acid at an elevated temperature, is diluted with water, and is filtered or centrifuged, if necessary, to remove any undissolved matter. Aluminium salts purified by this method are suitable for use as water treatment and retention chemicals.
The applicant's previous application PCT/FI95/00272 discloses an extraction-based method for purifying aluminium oxide. According to the method, aluminium oxide is pulverized and is thereafter dissolved in an acid at an elevated temperature. Thereafter the solution is diluted with water, and an organic solvent is added to it to extract the organic impurities. The phases are separated, the aluminium-salt-containing aqueous phase which has settled at the bottom is filtered or centrifuged to remove any residual undissolved matter and is used as such or is purified further. Part of the organic phase is directed to combustion and part for reuse as a solvent in the extraction step. Aluminium salts purified by this method are suitable for use as water treatment and retention chemicals.
Both of the methods described above have their disadvantages. The disadvantage of the method based on heat treatment is that the impurities become distilled off as a tarry substance in the combustion and thereby substantially inconvenience the combustion. A second disadvantage is that in the combustion the solubility of aluminium oxide in acid is reduced. Furthermore, the combustion process is expensive and produces emissions hazardous to the environment. The main disadvantage of the method based on extraction is also the formation of tarry substances, which adhere to the extraction apparatus and thereby hamper the extraction. Furthermore, as a process the extraction method is relatively complicated, and therefore the method is expensive. In addition, the method based on extraction also requires a combustion plant.